Bangkok, Thailand — A four-week military offensive in eastern Burma (Myanmar) has pushed back ethnic Karen rebels and forced thousands of refugees to flee across the border into Thailand. The attacks appear to underscore the determination of Burma's regime to snuff out what little armed opposition remains to its rule ahead of elections next year.

Burma is already in the international spotlight over its treatment of detained opposition leader Aung San Suu Kyi, who is on trial for breaking the terms of her house arrest after an American man swam across a lake to visit her against her will. The trial is due to resume on July 3.

Burma is also the presumed customer for the cargo aboard a North Korean ship that the US Navy is currently tailing. Burmese state media reported Thursday that authorities had no information on the vessel.

In recent months, Burma's junta has begun pressuring ethnic insurgent groups with which it has previously signed cease-fire agreements to put their fighters under military command as border guards. Most are expected to contest the elections, showing a willingness to participate in the process. But they have been reluctant to disarm after decades of strife with a military that is dominated by the ethnic Burman majority.

The largest group that has signed a cease-fire, the United Wa State Army (UWSA), has already rejected the regime's proposal. The UWSA has around 20,000 fighters and is accused of involvement with drug trafficking in northern Burma, which is second only to Afghanistan in opium production.

"It's a gamble. The ethnic groups are not falling into line," says a Western diplomat in Bangkok.

Karen militia under pressure

The Karen National Union (KNU) is among a handful of groups in Burma that haven't signed cease-fire agreements with the regime. Its armed wing, the Karen National Liberation Army (KNLA), has been fighting for six decades after Britain, the former colonial power, failed to deliver on a promise of self-rule for Karen people, who are estimated to number around seven million.

Burmese military offensives against the rebels normally intensify during the dry season. But, in a change of tactics, the current onslaught comes during monsoon rains that slow the advance of troops.

Since early June, Burmese troops, supported by a breakaway Karen militia, have forced the KNLA to abandon several bases along the border with Thailand. This has disrupted supply routes and raised fears of a wider collapse in its defenses that may trigger a larger refugee outflow.

More than 4,500 villagers have crossed the border to escape the fighting and out of fear of being abused by soldiers, says Zipporah Sein, general secretary of the KNU. More than 1,000 are staying at a Buddhist monastery, and others are sheltering with relatives or at makeshift camps.

Karen leaders are talking to Thai authorities and the UN's refugee agency about what do next. More than 140,000 mostly Karen refugees already live in camps along the border. "If it's not safe for people to return, they would be sent to these camps," says Ms. Sein.

Thai diplomats have said privately that the timing of the offensive is a deliberate rebuke to Thailand for its criticism of Ms. Suu Kyi's trial. By stirring unrest along the border, Burma may seek to warn Thailand, its historical rival, not to meddle in its political affairs.

In May, the Association of Southeast Asian Nations (ASEAN), to which Burma belongs and which Thailand currently chairs, warned that the trial was affecting the "honor and credibility" of Burma. The statement issued in Thailand's name provoked an angry response from Burma's state media and claims of interference by its neighbor. In the past, ASEAN has shied away from frank comment on member countries.

Burma is "sending a clear message to Thailand and other ASEAN countries to take them seriously. This [trial] is much more important for them than [defeating] the KNU," says Bo Hla Tint, a spokesperson for the National Coalition Government of the Union of Burma, an exiled group.

Western diplomats say Thailand doesn't want Burma's political impasse to overshadow a regional summit that it is hosting next month, which US Secretary of State Hillary Rodham Clinton is expected to attend. In April, the summit was called off at the last minute due to political unrest in Thailand.

Hobbled by infighting, the KNLA appears to be on the ropes, outgunned and outnumbered. Supporters say that combatants organized into conventional military units may eventually return to guerrilla warfare, as it becomes harder to hold territory. Political leaders could also sue for peace, though that would open more fissures in the movement.

"In the long run, we don't see how the KNU can hold on. They've been losing their camps over the last decade," says a Thai intelligence official.